This invention relates to a fluid pressure detector for measuring total pressure and static pressure of fluid flowing in a duct, and the invention also relates to an air flow rate measuring apparatus using this fluid pressure detector.
There is known a fluid pressure detector of the type in which a total pressure-measuring tube of a round (circular) cross-section and a static pressure-measuring tube of a round cross-section are connected together in parallel relation to each other, and a plurality of total pressure-measuring holes are formed in an upstream end of the total pressure-measuring tube, and a plurality of static pressure-measuring holes are formed in a downstream end of the static pressure-measuring tube. Such a fluid pressure detector is disclosed, for example, in Japanese Patent Unexamined Publication No. 3-220421 and Japanese Utility Model Unexamined Publication No. 60-135666.
In such a fluid pressure detector constituted by a combination of round measurement tubes, a pressure detection portion itself has poor ability to rectify or correct the flow, and it is difficult to detect the pressure in a stable manner, and particularly in the it is preferred that the flow should be rectified as much as possible at the total pressure-measuring position and the static pressure-measuring position.
In view of this, there has been proposed a fluid pressure detector (Japanese Patent Examined Publication No. 1-26010) which is formed into a generally-flattened shape. In this detector, a partition member is fixedly mounted within a flattened, hollow detector body, and is disposed perpendicular to a flow direction, thereby dividing the interior of this hollow body into two chambers, that is, an upstream-side chamber and a downstream-side chamber, and static pressure-detecting holes are formed in a direction perpendicular to the flow direction, and projections are provided downstream of the static pressure-detecting holes. Therefore, this detector has problems that the direction of mounting of a detector member is limited by the flow direction, and that a material, of which the detector is made, is limited because of its complicated construction, so that the use of this detector is limited.
There is known an air flow rate measuring apparatus of the type in which a flow rectifier is provided at an upstream-side portion of a casing, and a fluid pressure detector, such as a Pitot tube, is provided at a downstream-side portion of the casing (see Japanese Utility Model Unexamined Publication No. 60-80316). measurement under a deflection flow, an error is liable to occur in the detected pressure. Therefore, this fluid pressure detector is limited to a narrow range of use under a condition in which little deflection flow is involved. In the type of pressure detector in which projections are provided for the purpose of producing a stable eddy, the eddy is produced downstream of the projections over a wide range of flow velocity, and a constant pressure coefficient can be obtained regardless of a change in wind velocity (flow velocity). However, it has been proven that a constant pressure coefficient can be kept in a practical flow velocity range even without such projections. And besides, there is encountered a problem that an energy loss (pressure loss) of the fluid is increased because of the provision of the projections.
Furthermore, the power for conveying the fluid must be increased because of the increased energy loss, which results in a problem that a device, such as an air blower, must be increased in size, so that the running cost is increased.
A length of a mounting portion of the pressure detector is small in the direction of flow of the fluid, and therefore the pressure detector is liable to be mounted with an angle of attack relative to the flow, which results in a problem that an error in the detected pressure becomes large.
In the fluid pressure detector of this kind,
In the conventional air flow rate measuring apparatus, static pressure-measuring holes of the fluid pressure detector are formed in a direction perpendicular to the direction of flow of a fluid, and therefore it is required that the measurement should be carried out under a condition in which a stable flow with no eddy is present. Therefore, in order to avoid the influence of an eddy produced by the flow rectifier disposed at the upstream-side region, it is necessary to secure a sufficiently-long, straight tubular portion between the flow rectifier and the fluid pressure detector.
Generally, for the purpose of detecting a true static pressure in a flow passage, static pressure-measuring holes of a fluid pressure detector are formed in a direction perpendicular to the direction of flow of the fluid. Therefore, if a differential pressure detector is provided immediately after a flow rectifier as described above, there is encountered a great influence of an eddy produced immediately after the flow rectifier. This invites a problem that the measurement accuracy of the detected pressure is adversely affected.
In view of the foregoing, in a method of testing and inspecting an air blower according to Japanese Industrial Standards (JIS B 8330), the distance between a flow rectifier and a fluid pressure detector is larger than the bore (inner diameter) of a casing. This means that an air flow rate measuring apparatus, containing a flow rectifier therein, has an increased length in a direction of flow of fluid, which results in a problem that the apparatus is inevitably increased in size, so that a space, required for installing the apparatus, becomes large.